ESTRO 36 Abstract Book

S917 ESTRO 36 2017 _______________________________________________________________________________________________

minutes and 2 minutes, respectively. Three x-ray energies were used including 10MV, 25MV and 45MV. The radiation dose ranged from 1.0Gy to 10.0Gy per treatment. The dose distribution can be calculated from the activity distribution of 11 C and 15 O. Results It was confirmed that no activity was detected at 10 MV beam energy, which was far below the energy threshold for photonuclear reactions. At 25 MV x-ray beams could produce photonuclear reactions and activity distribution images were observed on PET. But it needed much higher radiation dose in order to obtain good quality images. For 45 MV photon beams, good quality activation images were obtained with 2-3Gy radiation dose, which is the typical daily dose for radiation therapy. Conclusion The PET image and the activity distribution of 15 O and 11 C positron emitter nuclei could be used to derive the dose distribution of 45MV x-ray irradiation at the regular daily dose level. This method can potentially be used to verify in situ dose distributions delivered to the patient using the LA45 accelerator. EP-1700 Prognostic value of FCH PET/CT in response to radical radiotherapy for localized prostate cancer M. Sepulcri 1 , L. Evangelista 2 , M. Fusella 3 , S. Galuppo 1 , L. Corti 1 , G. Saladini 2 , M. Paiusco 3 1 Veneto Institute of Oncology IOV-IRCCS, Radiation Oncology Unit, Padua, Italy 2 Veneto Institute of Oncology IOV-IRCCS, Nuclear Medicine and Molecular Imaging Unit, Padua, Italy 3 Veneto Institute of Oncology IOV-IRCCS, Medical Physics Unit, Padua, Italy Purpose or Objective To assess the value of FCH PET/CT in predicting the outcome of patients with localized prostate cancer treated by radical radiotherapy. Material and Methods From a mono-centric PET/CT database, we retrospectively reviewed pre-treatment FCH PET/CT scans of 24 patients who underwent radiotherapy for the treatment of localized prostate cancer. For each study, SUVmax, SUVavg and metabolic tumor volume (MTV) were evaluated. Moreover, the value of PSA before radiotherapy (PSAp) was recovered. Regarding radiation therapy, all patients underwent to a radical treatment for a total equivalent dose of 78-80 Gy, reached with a standard fractionation (2 Gy/fraction) or with an hypofractionated schedule (2.5 Gy/fraction). A follow-up period after PET/CT scan, of at least one-year, was required. In accordance with the observational period, patients were classified as disease free (DF) if the increase of PSA value after radiotherapy was less than 2 ng/mL respect to PSA nadir value, conversely with an increase of PSA higher than 2 ng/ml they were classified as recurrent (not disease free, NDF). A Kolgomorov-Smirnov test was used to compare the distribution of semi-quantitative PET and PSA data of the two patient groups. For all patients a simulated plan with a dose escalation on intraprostatic dominant lesion (IDL) was made. Results Mean, minimum and maximum values of SUVmax, SUVavg, MTV and PSAp were 9 (3.1-29.6), 4.2 (3.1-9.4), 13.3 (0.1- 49.1) and 18.3 ng/mL (4.5-88.7 ng/mL), respectively. After one year of follow-up, 20 patients were considered as DF and 4 patients were considered as NDF. The values of DF patients were 8.2 (3.1-29.6), 3.9 (3.1-6.7), 11.6 (0.1-29.7) and 16 ng/mL (4.5-54.8 ng/mL) respectively for SUVmax. SUVavg, MTV and PSAp. For NDF patients the corresponding obtained values were 14.3 (8.7-22.7), 6.4 (3.8-9.4), 24.8 (7.1-49.1) and 33.7 ng/mL (8.7-88.7 ng/mL). In NDF patients, the mean values of SUVmax and SUVavg were significantly higher than in DF group (both

p<0.05, fig.1) while MTV and PSAp were not statistically different between the two groups. This data, in accordance with the well known radiobiology of prostate cancer, suggest to increase the dose to the tumor. The analysis of OAR's DVH (bladder and rectum) showed that there are no significant changes between the standard treatment and the simultaneous integrated boost (SIB) approach, reaching a total dose to IDL volume around 105 Gy.

Conclusion High values of FCH SUV's in prostate cancer for patients who are candidates to radiotherapy result predictive of poor outcome after one year of follow-up. Therefore, the SUV values could be useful to identify those patients who could benefit from a boosted radiotherapy dose to the intraprostatic dominant tumor lesion. EP-1701 FDG-PET Background Definition in Rectal Cancer Patients Using Differential Uptake Volume Histograms J. Schneider 1 , N. Tomic 1 , T. Vuong 1 , R. Lisbona 2 , M. Hickeson 2 , G. Chaussé 2 , F. DeBlois 1 , J. Seuntjens 1 , S. Devic 1 1 McGill University, Oncology, Montreal, Canada 2 McGill University, Diagnostic Radiology, Montreal, Canada Purpose or Objective According to Erdi et al. [Cancer 1997;80:S2505-9] signal to background ratio (S/B) reflects the activity specific for local normal tissue, rather than making an assumption the activity is uniformly distributed over the whole body and recommended S/B as a quantity of choice for radiotherapy target definitions. In the case of paired organs (lung) Devic et al. [Int J Rad Oncol Biol Phys 2010; 78: 1555-62] sampled background uptake in contra-lateral healthy lung and scaled it by physical densities to obtain S/B for NSLC patients. Material and Methods Differential uptake volume histogram (dUVH) method [Devic et al. BJR 2016;89:20150388] was used on a group of 20 rectal adenocarcinoma patients that received pre- operative endorectal brachytherapy [Vuong et al, J Cont Brachyther 2015;7:183-8]. All patients had PET/CT scan prior to brachytherapy for staging purposes. Based on post-surgery pathology results half of the patients had complete response after brachytherapy (pT0) while the other half had no or minimal response. Uptake values (in Bq/ml) were sampled on PET images, using CT, and co- registered PET/CT images (Fig.1 top) by placing the sampling region of interest (ROI) over both tumor and healthy rectal tissue, and at the same time by avoiding air (gas) and feces.